The Planets of the Solar System
By far most of the solar system's mass is in the Sun itself: somewhere between 99.8 and 99.9 percent. The rest is split between the planets and their satellites, and the comets and asteroids and the dust and gas surrounding our star. Seen from afar (on the scale of distances between stars) the presence of the solar system would not be obvious. We would simply see a normal-looking star. Perhaps we would pick up the presence of Jupiter, which makes up two thirds or so of the solar system outside of the Sun, by mass.
Because Jupiter has most of the mass of what the Sun left over, we can think of the solar system as a double star with very unequal partners. The little guy (Jupiter) is 1/1000 the mass of the big one (Sun) and doesn't have enough mass to produce the pressure necessary for ignition at its center.
The Sun and the five largest planets of the solar system (to scale). The Earth is the tiny dot between Jupiter and the Sun. (Source: NASA)
It does have pretty much the same composition as the Sun (hydrogen and helium in solar proportions, along with some carbon, nitrogen and oxygen). Also, it does radiate heat — almost as much again as it receives from the Sun. This heat is released by the radioactive decay of elements within the planet and (perhaps) from gravitational effects, as the heavier elements move toward the center. (Earth also radiates its own heat, but it is negligible compared with energy of solar origin.)
One way of detecting Jupiter (from a far away solar-type system), would be from the fact that the Sun rotates around the common center of gravity, which is outside the Sun by more than one solar diameter. When the two big planets — Jupiter and Saturn — are aligned, the common gravity center ("barycenter") is two diameters away from the center of the Sun. Thus, the fact that the Sun has two large planets should be detectable as long as the small proper motion of the Sun around the barycenter can be resolved.
Jupiter (Source: NASA)
Jupiter and Saturn together make up more than 90 percent of the mass of all planets. The bulk of their mass is in hydrogen and helium, as mentioned. Also, the compounds ammonia (nitrogen hydride), methane (carbon hydride) and water (oxygen hydride) are seen in their atmospheres. These are the most obvious combinations of the four most common elements (besides helium), and the fact that they are abundant reflects the relatively low temperature of the outer atmosphere of the two main planets. (Helium, as a noble gas, does not combine with other elements.)
The next two largest planets are Uranus (discovered 1781, by William Herschel) and Neptune (discovered 1846, by Johannes Galle; see also Adams and Leverrier). The two are roughly equal in size and mass, at about five percent of Jupiter and less than a fifth of Saturn. The composition of Uranus is much like that of Jupiter and Saturn. Also, like those two giants, it has (wispy) rings and lots of satellites.
Saturn (Source: NASA)
Apparently, Neptune was first seen by Galileo (he sketched it when observing Jupiter in 1613) but he did not recognize its nature as a planet. Neptune has a composition much like the other "Jovian" planets, but has somewhat greater density. It too has debris and satellites encircling it. Pluto, the outermost planet, is roughly the size of the Moon and consists of dusty ice, much like a comet. It has a large companion, "Charon".
Going toward the Sun from Jupiter we find a totally different assemblage of planets. The biggest of the "inner" or "terrestrial" planets is Earth itself, at about 0.3 percent of the mass of Jupiter the 5th largest of the nine planets. Its atmosphere is quite unlike that of Jupiter and the outer planets in general. Instead of hydrogen and helium there is nitrogen and oxygen.
All the inner planets — Mercury, Venus, Earth and Mars — have solid surfaces with a sharp separation from their atmosphere. The solid matter consists of compounds of silicates ("minerals") rich in the elements magnesium, iron, calcium, sodium and potassium, besides the silicon and oxygen which provides the bulk for the various crystals. To account for the great density of the inner planets (more than 4 times higher than that of Jupiter) one must assume much enrichment with iron, over other elements.
Uranus (Source: NASA)
(Iron is what makes Mars red, and links it to blood, which is spilled in war; hence Mars has the name of the ancient war god.)
As far as the atmospheres of the inner planets, that of Earth is highly unusual, because it is chemically unstable (nitrogen and oxygen combine within lightning).
This instability is a give-away for the presence of Life. Mercury has no atmosphere. Its gravity is too low to hold on to gas heated by the Sun. (The same is true for the Moon, a body much like Mercury). Venus has an atmosphere almost a hundred times more massive than that on Earth. Its pressure corresponds to that in the ocean at about 1 km depth. But it is not water, it is carbon dioxide! At the surface it is hot enough to melt lead. Hydrochloric and sulfuric acids also are present. Only two percent of the sunlight penetrates to the surface. Most of it is reflected.
Neptune (Source: NASA)
(The high reflectivity, which is due to sulfuric clouds, makes Venus the brightest object in the sky, after the Sun and the Moon. Its beauty is recognized in its name.)
The atmosphere of Mars has but one-hundredth the pressure of that of Earth. It also is almost entirely carbon dioxide, with some nitrogen and argon.
The reason that the atmosphere of Mars is so thin is that the gravity of Mars, the smallest of the inner planets, is relatively weak.
In any event, our companion planets Venus and Mars have atmospheres made of carbon dioxide, while in our atmosphere carbon dioxide is but a trace gas. The reason is that life processes have removed the carbon from the atmosphere into limestone rocks, coal, oil and gas (in the ground and as methane ice). Limestones are made of carbonate fossils, originally. Coal is finely disseminated, mostly, in sedimentary rocks, and made from plant materials. Oil and gas are made from marine organic matter, originally.
Pluto The brown color of the planet is thought to be due to frozen methane deposits. No spacecraft from Earth has as yet visited Pluto. This image was taken with an Earth-based telescope. (Source: NASA)
We are now using coal, oil and gas for production of energy (for the time reversing the long-term extraction of carbon from the atmosphere). Thus, carbon dioxide (and methane) are growing in abundance in Earth's atmosphere. Also, we are using limestones to make cement, which makes a modest contribution to carbon dioxide, as well. The recent change in the chemistry of the atmosphere of Earth is perhaps the most significant indicator of the presence of intelligent life in the solar system. (The emission of radio-waves carrying news about football games and stock prices is a close second.)
Mercury (Source: NASA)
Mars (Source: NASA)
Venus (Source: NASA)